At present, wind energy power conversion systems which utilize horizontal axis wind turbines with an objective of obtaining stable electrical power from natural wind have been proposed and put into practical use. As shown in FIG. 7 and FIG. 8, these horizontal axis wind turbines are comprised of a rotor that has a hub 21 and a blade 22, a nacelle 24 that axially supports the rotor through a main shaft 23 that extends in the approximate horizontal direction connected to the hub 21, and a tower installed in an approximate vertical direction along with supporting the nacelle 24. In addition, a spinner 31, that covers the hub 21, is also attached. As shown in FIG. 8, a wind energy power conversion system that utilizes a horizontal axis wind turbine is equipped with a generator 27 connected to the main shaft 23 inside the nacelle 24 through a step-up gear 26 to form the arrangement of a wind energy power conversion system and generates electrical power by receiving wind and converting the rotational energy of the rotating rotor to an electrical energy. Conventionally, as shown in FIG. 9, a main shaft connecting part 29 on a spherical shell 28 and a circular flange 30 used to secure a blade are used as the hub 21 of a horizontal axis wind turbine. The inside of the circular flange 30 is open. A base assembly of a blade is secured to this circular flange 30. Casting is applied as the method to manufacture the hub.
PCT Publication No. WO2004/090326 A1 (hereinafter referred to as “Patent Reference 1”) and PCT Publication No. WO2003/064854 A1 (hereinafter referred to as “Patent Reference 2”) both describe a composition formed with a reinforcement plate that covers one part of the inside of a circular flange without opening the entire inside of a circular flange used to secure a blade.
The reinforcement plate described in Patent Reference 1 is formed in a bridge shape extending radially in two to four directions from the center of a circular flange used to secure a blade and is coupled to a spherical shell at a position inside a circular flange. The reinforcement plate described in Patent Reference 2 is formed in a shape that covers the peripheral edge of a circular region inside a circular flange and the center area is opened.
This type of hub that has a reinforcement plate inside a circular flange used to secure a blade can ensure sufficient strength even if the casing thickness of the spherical shell is thin. This makes it possible to reduce the weight of the hub.
In addition, by means of making the casing thickness of the hub thinner, the inherent segregation of the casting (which occurs more easily when the casing thickness is thicker) can be reduced making it possible to improve the quality.
The following problems exist for the conventional technology described above. Casting methods are adopted in which molten metal enters from the lower area of a casting mold, rises inside the hub molding space, and reaches the exhaust area at the upper portion, while casting a hub.
This type of casting method has the following different properties at the lower area on the inlet side and the upper area on the exhaust side.
At first, the lower area on the inlet side solidifies and then the upper area on the exhaust side solidifies. Compared to the lower area on the inlet side, segregation or microscopic shrinkage (micro-porosity) easily occurs on the upper area on the exhaust side. Because a thinner casing of the casting makes it more difficult for defects to occur while casting such as segregation or microscopic shrinkage, the casing thickness is designed to be thinner higher up in the upper area on the exhaust side. Nevertheless, compared to the lower area on the inlet side, the upper area of the exhaust side has a disadvantage in terms of strength.
Because a high strength is required on the main shaft connection side for the hub, the main shaft connection side of the hub must be established on the lower area of the casting mold.
If however, the main shaft connection side of the hub is established on the lower area of the casting mold, the main shaft connection side of the hub will have a relatively high strength and the tip of the hub will have a low strength resulting in an imbalance.
In the hub in Patent Reference 1, the reinforcement plate extends from the center of the circular flange used to secure a blade to the main shaft connection side and is coupled to the spherical shell at a position inside the circular flange. In other words, considering the reinforcement of the main shaft connection side, relatively speaking, it is not obvious to reinforce the tip of the hub.
Because of this, there is a chance that the imbalance in strength in the reinforcement plate in Patent Reference 1 may not be sufficiently cancelled.
In addition, since the main shaft connection side of the hub is formed with a casing thickness of greater thickness, the coupling of the reinforcement plate to the main shaft connection side of the hub is a drawback from the viewpoint of lighter weight.
In the hub in Patent Reference 2, the reinforcement plate is comprised in a shape that covers the peripheral edge of the inside circular region of the circular flange and the reinforcement strength at the main shaft connection side of the hub and the tip are at the same level. The reinforcement plate is also coupled to the spherical shell at a position inside the circular flange at the main shaft connection side.
Therefore, the reinforcement plate in Patent Reference 2 cannot sufficiently cancel the imbalance in strength mentioned above.
Even further, since the casing thickness of the main shaft connection side of the hub is formed with a heavy thickness, the coupling of the reinforcement plate to the main shaft connection side of the hub is a drawback from the viewpoint of lighter weight.
The present invention takes the problems in the conventional technology mentioned above and provides a lightweight hub of a horizontal axis wind turbine with a balanced strength.